Method and system for imaging using multiple offset X-ray emission points

ABSTRACT

A technique is provided for imaging a field of view using an X-ray source comprising two or more emission points. Each emission point is configured to emit a fan of radiation encompassing less than the entire field of view. The emission points are activated individually and rotate about the field of view, allowing respective streams of radiation to be emitted at various view angles about the field of view. The emission points, which may correspond to different radial regions of the field of view, may be differentially activated to emphasize a region of interest within the field of view. The multiple emission points may be extrapolated along the longitudinal axis in duplicate or offset configurations.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.10/789,539, entitled “Method and System for Imaging Using MultipleOffset X-Ray Emission Points”, filed Feb. 27, 2004, which is hereinincorporated by reference in its entirety for all purposes.

BACKGROUND OF THE INVENTION

The present invention relates generally to the field of non-invasiveimaging and more specifically to the field of computed tomography (CT)imaging. In particular, the present invention relates to sourceconfigurations useful in CT imaging.

CT scanners operate by projecting fan-shaped or cone-shaped X-ray beamsfrom an X-ray source. The X-ray source emits X-rays at numerous viewangle positions about an object being imaged, such as a patient, whichattenuates the X-ray beams as they pass through. The attenuated beamsare detected by a set of detector elements, which produce signalsrepresenting the intensity of the incident X-ray beams. The signals areprocessed to produce data representing the line integrals of theattenuation coefficients of the object along the X-ray paths. Thesesignals are typically called “projection data” or just “projections”. Byusing reconstruction techniques, such as filtered backprojection, usefulimages may be formulated from the projections. The images may in turn beassociated to form a volume rendering of a region of interest. In amedical context, pathologies or other structures of interest may then belocated or identified from the reconstructed images or rendered volume.

It is generally desirable to develop CT scanners with high spatial andtemporal resolution, good image quality, and good coverage along thez-axis, i.e., the longitudinal axis of the CT scanner. To meet some orall of these objectives, it may be desirable to increase the coverageprovided by the detector, thereby allowing greater scan coverage in oneor more dimensions. For example, longitudinal axis coverage of thedetector may be improved by increasing the number of rows of detectorelements in the detector.

This approach has lead to the development of CT systems with largerdetectors. Larger detectors, however, may be undesirable for a varietyof reasons. For instance, as one might expect, larger detectors andassociated acquisition electronics are both more costly and moredifficult to produce. In addition, the mechanical subsystem responsiblefor supporting and/or rotating a larger detector may also need to belarger and more complex and/or may be subject to greater mechanicalstress. Furthermore, large detectors are associated with increased coneangles, i.e., the angle between the source and the detector periphery.The increased cone angle between the source and detector periphery is inturn associated with increased cone-beam artifacts in the reconstructedimages. When the cone angle increases beyond a certain limit, thedegradation of the image quality may become severe for axial, or stepand shoot scanning. For this reason, it may be difficult to increase thescan coverage by simply increasing the coverage, i.e., size of thedetector. A technique for achieving high spatial and temporalresolution, good image quality, and good coverage using a standard orsmaller detector may therefore be desirable.

BRIEF DESCRIPTION OF THE INVENTION

The present technique provides a novel method and apparatus forproviding two or more discrete X-ray emission points, which arelaterally offset, i.e., have different xy-coordinates. In particular,the sources are offset in an azimuthal direction such that each sourceprovides a particular subset of the projection lines needed toreconstruct the imaged object within the field of view. The sources maybe alternately activated, though not necessarily at equal intervals,i.e., some of the sources may be activated more frequently or forgreater duration than others. A single detector may be employed inconjunction with the two of more sources. The detector may have arelatively small in-plane extent and may be a flat-panel detector insome implementations.

In accordance with one aspect of the present technique, a method isprovided for imaging a field of view. The method includes rotating anX-ray source about a field of view. The X-ray source may comprise two ormore, discrete emission points. At least two of the emission points areindividually activated at view angles around the field of view. Eachemission point, when activated, emits a respective stream of radiationthrough a respective portion of the field of view. A plurality ofsignals generated in response to the respective streams of radiation areacquired from a detector. The plurality of signals are processed togenerate one or more images. Systems and computer programs that affordfunctionality of the type defined by these methods are also provided bythe present technique.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other advantages and features of the invention willbecome apparent upon reading the following detailed description and uponreference to the drawings in which:

FIG. 1 is a diagrammatical view of an exemplary imaging system in theform of a CT imaging system for use in producing processed images, inaccordance with one aspect of the present technique;

FIG. 2 is an in-plane view of a pair of X-ray emission points in a fullfield-of-view configuration, in accordance with the present technique;

FIG. 3 is an in-plane view of a pair of X-ray emission points in a halffield-of-view configuration, in accordance with the present technique;

FIG. 4 is an in-plane view of a pair of X-ray emission points in anarbitrary field-of-view configuration, in accordance with the presenttechnique;

FIG. 5 is an in-plane view of four X-ray emission points in a fullfield-of-view configuration, in accordance with the present technique;

FIG. 6 is an in-plane view of four X-ray emission points in a halffield-of-view configuration, in accordance with the present technique;

FIG. 7 is an in-plane view of four X-ray emission points in an arbitraryfield-of-view configuration, in accordance with the present technique;

FIG. 8 is a perspective view of a CT scanner having a configuration ofemission points that are offset along the longitudinal axis, inaccordance with the present technique;

FIG. 9 is a side view of multiple axial X-ray emission points and adetector, in accordance with the present technique; and

FIG. 10 is a perspective view of a CT scanner having a duplicateconfiguration of emission points along the longitudinal axis, inaccordance with the present technique.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

FIG. 1 illustrates diagrammatically an imaging system 10 for acquiringand processing image data. In the illustrated embodiment, system 10 is acomputed tomography (CT) system designed to acquire X-ray projectiondata, to reconstruct the projection data into an image, and to processthe image data for display and analysis in accordance with the presenttechnique. Though the imaging system 10 is discussed in the context ofmedical imaging, the techniques and configurations discussed herein areapplicable in other non-invasive CT imaging contexts, such as baggage orpackage screening.

In the embodiment illustrated in FIG. 1, CT imaging system 10 includes asource 12 of X-ray radiation. As discussed in detail herein, the source12 of X-ray radiation may consist of two or more discrete, i.e.,separated, emission points. For example, a conventional X-ray tube maybe equated with a single emission point. Alternatively, an X-ray sourcesuch as a solid-state X-ray source having field emitters, or athermionic X-ray source may include multiple emission points. Suchsolid-state or thermionic X-ray sources may be configured such that therespective emission points form an arc or a stationary ring.

Though the present description may discuss the rotation of an X-raysource 12, as may occur in conventional third-generation CT systems, oneof ordinary skill in the art will appreciate that discussion of arotating an X-ray source 12 also encompasses functional equivalents. Forexample, for a solid-state X-ray source 12 configured as a ring, thesource 12 and respective emission points may not physically rotate.Instead, emission points along the ring may be activated in a sequentialmanner effectively equivalent to rotating an X-ray source 12. Therefore,where an X-ray source 12 or emission point is described as rotating, itis to be understood that such a rotation may result from the physicalrotation of the source 12 or elements of source 12 or from such afunctional equivalent.

The X-ray source 12 may be positioned proximate to a collimator 14. Thecollimator 14 may consist of a collimating region, such as lead ortungsten shutters, for each emission point of the source 12. Thecollimator 14 typically defines the size and shape of the one or morestreams of radiation 16 that pass into a region in which a subject, suchas a human patient 18, is positioned. A stream of radiation 16 may begenerally cone-shaped, depending on the configuration of the detectorarray, discussed below, as well as the desired method of dataacquisition. An attenuated portion of the radiation 20 passes throughthe subject, which provides the attenuation, and impacts a detectorarray, represented generally at reference numeral 22.

The detector 22 is generally formed by a plurality of detector elements,which detect the X-rays that pass through and adjacent to a subject ofinterest. The detector 22 may include multiple rows of detectorelements. When such multi-row detectors are employed, the stream ofradiation 16 will have a non-zero cone-angle associated with it fordetector rows not in-plane with the active emission point. The followingexamples may make abstraction of this z-extent to simplify presentation,i.e., by limiting discussion to the detector elements in-plane with theactive emission point. However, as one of ordinary skill in the art willappreciate, the following geometrical discussion and examples areequally applicable to multi-row detectors.

Each detector element, when impacted by an X-ray, produces an electricalsignal that represents the intensity of the X-ray beam at the positionof the element during the time the beam strikes the detector. Typically,signals are acquired at a variety of view angle positions around thesubject of interest so that a plurality of radiographic views may becollected. These signals are acquired and processed to reconstruct animage of the features within the subject, as described below.

The X-ray source 12 is controlled by a system controller 24, whichfurnishes power, focal spot location, control signals and so forth forCT examination sequences. Moreover, the detector 22 is coupled to thesystem controller 24, which commands acquisition of the signalsgenerated in the detector 22. The system controller 24 may also executevarious signal processing and filtration functions, such as for initialadjustment of dynamic ranges, interleaving of digital image data, and soforth. In general, system controller 24 commands operation of theimaging system 10 to execute examination protocols and to processacquired data. In the present context, system controller 24 alsoincludes signal processing circuitry, typically based upon a generalpurpose or application-specific digital computer, and associated memorycircuitry. The associated memory circuitry may store programs androutines executed by the computer, configuration parameters, image data,and so forth. For example, the associated memory circuitry may storeprograms or routines for implementing the present technique.

In the embodiment illustrated in FIG. 1, system controller 24 maycontrol the movement of a rotational subsystem 26 and linear positioningsubsystem 28 via a motor controller 32. In imaging system 10 in whichthe source 12 and/or the detector 22 may be rotated, the rotationalsubsystem 26 may rotate the X-ray source 12, the collimator 14, and/orthe detector 22 through one or multiple turns around the patient 18. Itshould be noted that the rotational subsystem 26 might include a gantry.The linear positioning subsystem 28 enables the patient 18, or morespecifically a patient table, to be displaced linearly. Thus, thepatient table may be linearly moved within the gantry to generate imagesof particular areas of the patient 18.

As will be appreciated by those skilled in the art, the source 12 ofradiation may be controlled by an X-ray controller 30 disposed withinthe system controller 24. The X-ray controller 30 may be configured toprovide power and timing signals to the X-ray source 12. In addition,the X-ray controller may be configured to provide focal spot location,i.e., emission point activation, if the X-ray source 12 is a distributedsource, such as a solid-state or thermionic X-ray source configured asan arc or ring.

Further, the system controller 24 may comprise a data acquisition system34. In this exemplary embodiment, the detector 22 is coupled to thesystem controller 24, and more particularly to the data acquisitionsystem 34. The data acquisition system 34 receives data collected byreadout electronics of the detector 22. In particular, the dataacquisition system 34 typically receives sampled analog signals from thedetector 22 and converts the data to digital signals for subsequentprocessing by a computer 36.

The computer 36 is typically coupled to the system controller 24. Thedata collected by the data acquisition system 34 may be transmitted tothe computer 36 for subsequent processing and reconstruction. Forexample, the data collected from the detector 22 may undergopre-processing and calibration at the data acquisition system 34 and/orthe computer 36 to condition the data to represent the line integrals ofthe attenuation coefficients of the scanned objects. The processed data,commonly called projections, may then be reordered, filtered, andbackprojected to formulate an image of the scanned area. Oncereconstructed, the image produced by the system of FIG. 1 reveals aninternal region of interest of the patient 18 which may be used fordiagnosis, evaluation, and so forth.

The computer 36 may comprise or communicate with a memory 38 that canstore data processed by the computer 36 or data to be processed by thecomputer 36. It should be understood that any type of computeraccessible memory device capable of storing the desired amount of dataand/or code may be utilized by such an exemplary system 10. Moreover,the memory 38 may comprise one or more memory devices, such as magneticor optical devices, of similar or different types, which may be localand/or remote to the system 10. The memory 38 may store data, processingparameters, and/or computer programs comprising one or more routines forperforming the processes described herein.

The computer 36 may also be adapted to control features enabled by thesystem controller 24, i.e., scanning operations and data acquisition.Furthermore, the computer 36 may be configured to receive commands andscanning parameters from an operator via an operator workstation 40which may be equipped with a keyboard and/or other input devices. Anoperator may thereby control the system 10 via the operator workstation40. Thus, the operator may observe the reconstructed image and otherdata relevant to the system from computer 36, initiate imaging, and soforth.

A display 42 coupled to the operator workstation 40 may be utilized toobserve the reconstructed image. Additionally, the scanned image may beprinted by a printer 44 which may be coupled to the operator workstation40. The display 42 and printer 44 may also be connected to the computer36, either directly or via the operator workstation 40. Further, theoperator workstation 40 may also be coupled to a picture archiving andcommunications system (PACS) 46. It should be noted that PACS 46 mightbe coupled to a remote system 48, radiology department informationsystem (RIS), hospital information system (HIS) or to an internal orexternal network, so that others at different locations may gain accessto the image data.

One or more operator workstations 40 may be linked in the system foroutputting system parameters, requesting examinations, viewing images,and so forth. In general, displays, printers, workstations, and similardevices supplied within the system may be local to the data acquisitioncomponents, or may be remote from these components, such as elsewherewithin an institution or hospital, or in an entirely different location,linked to the image acquisition system via one or more configurablenetworks, such as the Internet, virtual private networks, and so forth.

The CT imaging system 10 described above may be configured in a varietyof ways to improve spatial and temporal resolution, to improve imagequality, and/or to improve longitudinal coverage. Indeed, various source12 and detector 22 configurations may be implemented which improve oneor more of these parameters. For example, as discussed herein, an X-raysource 12 that employs multiple emission points may be employed.Activation of the emission points may be coordinated so that only one isactive at a time, such as by employing an alternating activation scheme.In this manner, each emission point, when active, may provide a subsetof the projection lines required to reconstruct an object within a givenfield of view. Combination of these subsets, however, allows thereconstruction of the field of view. In addition, because only a subsetof the projection lines associated with the field of view are acquiredat one time, the in-plane extent of the detector 22 may be reduced.Indeed, the in-plane extent of the detector 22 may be reduced to thedegree that a flat-panel detector may be employed.

As one of ordinary skill in the art will appreciate, a variety of X-raysource 12 configurations and activation schemes may be practiced inaccordance with the present technique. A number of exemplaryconfigurations and schemes are discussed herein. It is to be understood,however, that the included examples do not limit the scope of thepresent technique. Instead, the present technique may broadly beunderstood to encompass any X-ray source configuration that allows formultiple, discrete emission points as well as any activation scheme forsuch emission points.

For example, as depicted in FIG. 2, a pair of discrete emission points70 offset in an azimuthal direction are depicted in an %-plane, as thesource 12 of radiation. The emission points 70 may be configured to bethe same perpendicular distance from the detector 22, such as flat-paneldetector 60, or may be different distances. Each emission point 70 maybe an X-ray tube, an emitter of a solid-state or thermionic X-raysource, or some other focal point from which X-rays may be emitted whenactivated. The X-ray source 12, and its respective emission points 70,may be gridded. The emission points 70 may also be offset in z, asdiscussed later in more detail.

The emission points 70 may be rotated about the desired field of view72, allowing each emission point 70 to emit streams of radiation 16 fromthe desired view angles. As the emission points 70 rotate, they may bealternatingly activated such that only one emission point 70 emitsX-rays at a given time. Each emission point 70 may be configured to emita fan-shaped stream of radiation when activated, which circumscribes aportion of the field of view 72, such as half the field of view 72 asdepicted in FIG. 2. The stream of radiation 16 passes through the fieldof view 72, and any attenuating matter within the field of view 72,before striking the detector 22, such as flat-panel detector 60. Foreach activation of an emission point 70, the data acquisition system 34(FIG. 1) reads out the signals generated by the detector 22, which maybe processed to generate the projection data. As the emission points 70rotate about the field of view 72 the combined or aggregate acquiredprojection data describes the entire field of view.

For example, a first emission point 74, when active, may emit X-rayswithin a fan encompassing a portion of the field of view 72, such ashalf the field of view 72, as depicted in FIG. 2. Projection data may,therefore, be acquired for this portion by the detector 22, such asflat-panel detector 60, when the first emission point 74 is active. Whenthe first emission point 74 is inactive, the second emission point 76may be activated, allowing projection data to be acquired for a portionof the field of view 72 encompassed by the fan of X-rays emitted bysecond emission point 76. The emission points 70 may be rotated aboutthe field of view 72, being alternatingly activated at each desired viewangle, until the desired projection data has been acquired toreconstruct the field of view 72.

As will be appreciated by one of ordinary skill in the art, sufficientprojection data to reconstruct the field of view 72 may be acquired withless than a full rotation of the emission points 70 about the field ofview 72. Indeed, a half rotation plus the angle (β) between the twoemission points 70, i.e., 180°+β, may be sufficient rotation to provideprojection data to reconstruct the field of view 72.

Furthermore, the multiple emission points 70 may be configured so thattheir combined fans circumscribe only half, or some other portion, ofthe field of view 72 when active, i.e., a half field of viewconfiguration. For example, referring to FIG. 3, two emission points 70are depicted which, when active, emit X-rays within a fan encompassingonly a portion of half of the field of view 72. The combined fans of thefirst and second emission points 74, 76, as depicted, circumscribe onlyhalf of the field of view 72. Limiting the fan angle, α, associated witheach emission point 70, allows the in-plane extent of the detector 22,here flat-panel detector 60, to be further reduced since less of thefield of view 72 is imaged when an emission point 70 is active. As oneof ordinary skill in the art will recognize, sufficient projection datato reconstruct the field of view 72 using a half field of viewconfiguration, as depicted in FIG. 3, may be acquired with a fullrotation of the emission points 70 about the field of view 72.

In addition, it should be recognized that the X-ray emitted by the firstemission point 74 and the second emission point 76 do not pass throughthe same regions of the field of view 72. In particular, the X-raysemitted by the first emission point 74 pass through the central regionof the field of view 72, where the imaged object or patient is typicallycentered. Conversely, the X-rays emitted by the second emission point 76pass through a peripheral region of the field of view 72, which maycontain empty space or regions of the imaged patient or object that areof less interest. This relationship remains true as the first and secondemission points 74, 76 rotate about the field of view 72, i.e., thefirst emission point 74 continues to image the central region of thefield of view 72 while the second emission point 76 continues to imagethe periphery of the field of view 72.

Because of this distinction between the first and second emission points74, 76, the first and second emission points 74, 76 need not be operatedequivalently, such as when the periphery of the field of view 72 is ofless or no interest. For example, fewer views may be acquired using thesecond emission point 76 if desired, i.e., the second emission point 76may be activated less frequently than the first emission point 74. Forinstance, the second emission point 76 may be activated for every otherview, or less, if desired. Similarly, the second emission point 76 maybe operated for a reduced duration or duty cycle, or at a lower energyrelative to the first emission point 74.

Likewise, the second emission point 76 may be of lower quality, i.e.,lower flux, and so forth than the first emission point 74, if theperipheral region imaged by the second emission point 76 is lessimportant. In particular, if lower attenuation, lower resolution, and/orhigher noise are acceptable for the periphery of the region of interest72, a lower flux second emission point 76 may be acceptable.Differential activation of the first and second emission points 74, 76and/or the use of a lower flux second emission point 76 may allowdifferent doses to be applied to the patient 18 at the center andperiphery of the region of interest 72. In this manner, the dosereceived by the patient 18 may be customized based on the circumstances.

These concepts may be extended to arbitrary configurations between ahalf and full field of view configuration or where a distinct centralregion of interest 80, such as a cardiac field of view, may be present.For example, as depicted in FIG. 4, the first and second emission points74, 76, may each circumscribe the different portions of the field ofview 72, i.e., the central region of interest 80 and the peripheralregion 82 respectively. As one of ordinary skill in the art willappreciate, the discussion of the central region of interest 80 andperipheral region 82 with regard to FIG. 4 is analogous to and expandsupon the related discussion with regard to FIG. 3.

In particular, referring to FIG. 4, the first emission point 74, whenactive, may emit X-rays within a fan encompassing the central region ofinterest 80 within the field of view 72. In this manner, the firstemission point 74 may generate the projection lines associated with thecentral region of interest 80. The second emission point 76, whenactive, may emit X-rays within a fan encompassing a radial or peripheralportion 82 of the region of interest 72 outside the central region ofinterest 80. For example, one edge of the fan of X-rays emitted by thesecond emission point 76 may be tangential to the central region ofinterest 80 and the other edge may be tangential to the edge of thefield of view 72. In this manner, the second emission point 76 maygenerate projection lines for a complementary portion of the field ofview 72 not contained within the central region of interest 80.

As with the preceding examples, because the entire field of view 72 isnot covered by a single emission point 70 and detector 22, the in-planesize of the detector 22 may be smaller than if a single emission point70 were employed. For example, the detector 22 may have a relativelysmall in-plane extent and, indeed, may be substantially flat, such asflat panel detector 60. For example, for a radius of the central regionof interest 80 of 15 cm and a radius of the field of view 72 of 50 cm,the detector 22 may be 30 percent or less of the size of a respectivedetector associated with the same field of view and a single emissionpoint 70.

Half-scan data acquisition may be used to acquire data forreconstructing the central region of interest 80, i.e., 180°+α degreesof rotation. Further, because the fan angle, α, is less than when asingle emission point 70 is employed, the half-scan may be performedmore rapidly, thereby providing improved temporal resolution for imagingdynamic organs such as the heart. For example, α may equal 15° insteadof 50° when a second emission point 76 is employed such that thehalf-scan data acquisition may encompass 195° of rotation of the firstemission point 74 instead of 230° degrees of rotation. However, a fullrotation, i.e., 360°, of the first and second emission points 74, 76 maybe needed to acquire data for reconstructing the full field of view 72,i.e., to fully reconstruct the peripheral region 82.

As noted above with regard to the half field of view configuration ofFIG. 3, fewer views using the second emission point 76 may be acquiredif desired, such as when the peripheral views supplied by the secondemission point 76 are less important. Similarly, the second emissionpoint 76 may be activated less frequently than the first emission point74 or for a reduced duration, as discussed in the preceding example.Likewise, as previously discussed, the second emission point 76 may beof lower quality, i.e., lower flux, and so forth than the first emissionpoint 74, if the peripheral region 82 imaged by the second emissionpoint 76 is less important.

Differential activation of the first and second emission points 74, 76and/or the use of a lower flux second emission point 76 may allowdifferent doses to be applied to the patient 18 inside and outside ofthe central region of interest 80. Indeed, in some instances, such aswhere the object or organ to be imaged is within the central region ofinterest 80, it may be possible to leave the second emission point 76inactive during image data acquisition. In such an implementation, thedata acquired corresponding to the peripheral region 82 will beincomplete, but may still be reconstructed using special reconstructiontechniques if desired, such as if some portion of the imaged object lieswithin the peripheral region 82. In this manner, the dose received bythe patient 18 may be customized based on the circumstances.

Though the preceding examples discuss implementations including twoemission points 70, the technique is extendable to three or moreemission points 70. For example, three or more X-ray tubes may beemployed or a solid-state or thermionic X-ray source 12 may be employedwhich includes three or more addressable emission points 70 configuredin an arc or ring. Other X-ray sources 12, which include discrete andaddressable emission points 70, may also be suitable for use with thepresent techniques.

For example, FIG. 5 depicts four emission points 70 in a fullfield-of-view configuration, analogous to that depicted in FIG. 2. Theemission points 70 may be configured to be the same perpendiculardistance from the flat-panel detector 60 or may be different distances.As discussed with regard to FIG. 2, the emission points 70 may berotated about the desired field of view 72 such that each emission point70 may emit a stream of radiation 16 from the desired view angles.

As the emission points 70 rotate, they may be alternatingly activatedsuch that only one emission point 70 emits X-rays at a given time. Eachemission point 70 may be configured to emit a fan-shaped stream ofradiation when activated, which circumscribes a portion of the field ofview 72. The stream of radiation 16 passes through the field of view 72,and any attenuating matter within the field of view 72, before strikingthe flat-panel detector 60. For each activation of an emission point 70,the data acquisition system 34 (FIG. 1) reads out the signals generatedby the detector 22, which may be processed to generate the projectiondata. As the emission points 70 rotate about the field of view 72 thecombined or aggregate acquired projection data describes the entirefield of view. As discussed above, in such a full field-of-viewconfiguration, sufficient projection to reconstruct the field of view 72may be acquired with a half-scan acquisition, i.e., 180°+ someadditional angle depending on the geometry.

Similarly, a half field of view configuration may be implemented usingmore than two emission points 70. For example, referring to FIG. 6, fouremission points 70 are depicted whose fan-shaped streams of radiation 16generally circumscribe half, or some other portion, of the field of view72. Each emission point 70 may be alternatingly activated, as describedabove, such that only one emission point 70 is active at a time. Due tothe limited fan angle, α, associated with each emission point 70, thedetector 22 may have a reduced in-plane extent. In such a half field ofview configuration, sufficient projection data to reconstruct the fieldof view 72 may be acquired with a full rotation of the emission points70 about the field of view 72.

Furthermore, as noted above, the emission points circumscribe differentradial regions of the field of view 72. For example, the first emissionpoint 74 defines a central region while the second emission point 76circumscribes the next outward radial region. Similarly, the thirdemission point 86 circumscribes the next radial region and the fourthemission point 88 circumscribes the peripheral or outer radial region.Because the emission points 70 circumscribe different radial regions ofthe field of view 72, different emission points 70 may remain inactiveduring an imaging sequence if the radial region they circumscribe is ofno or little interest. For example, the fourth emission point 88 mayremain inactive if the peripheral region of the field of view 72contains empty space or is otherwise of no interest. As with theprevious discussion of a half field of view configuration, sufficientprojection data to reconstruct the field of view 72 using a half fieldof view configuration, as depicted in FIG. 6, may be acquired with afull rotation of the emission points 70 about the field of view 72.

Similarly, and as discussed with regard to FIGS. 3 and 4, the first,second, third, and fourth emission points 74, 76, 86, 88 need not beoperated equivalently to the extent that the different radial regionsthey circumscribe are of different interest or importance. For example,each emission point 70 may be active for different numbers of views. Forexample, the first and second emission points 74, 76 may be active forevery view, the third emission point 86 may be active for every otherview, and the fourth emission point 88 may not be active for any view.Such an implementation might allow images to be constructed with goodquality toward the center of the field of view, less quality outside ofthe center, and with no image of the peripheral region of the field ofview 72 being generated. Similarly, different emission points, such asthe fourth emission point 88, may be operated for a reduced duration orat a lower energy relative to the first emission point 74. Likewise,emission points 70 may vary in quality, i.e., flux, based on the radialregion they circumscribe. For example, in an X-ray tube implementation,the third and/or fourth emission points 86, 88 may be low quality, i.e.,low flux, X-ray tubes.

Therefore, as the number of X-ray emission points 70 increases, theability to adapt the X-ray dose to the patient 18 or imaged object mayalso increase. In particular, possible number of radial regionsincreases as the number of emission points 70 increases. As the numberof radial regions increases, the opportunities to employ differentialoperation, such as activations and/or durations, or different hardwareconfigurations, such as low-flux X-ray tubes, also increases. In thismanner, the dose received by the patient 18 and the image quality indifferent portions of the image may be customized based on thecircumstances.

Likewise, the use of additional emission points 70 may be extended toarbitrary configurations or to configurations with a distinct centralregion of interest 80, such as a cardiac field of view 80, as discussedwith regard to FIG. 4. For example, referring to FIG. 7, the first andsecond emission points 74, 76 may circumscribe the central region ofinterest 80 of the field of view 72. Conversely, the third and fourthemission points 86, 88 may circumscribe the peripheral region 82 of thefield of view 72. The emission points 70 may be differentially operatedor constituted, as discussed with regard to FIGS. 4 and 6, such thatpatient dosage may be adapted or adjusted based on circumstance. Forexample, the third and/or fourth emission points 86, 88 may not beactivated or may be activated for only a subset of the possible viewangles when the peripheral region 82 is of less or no interest.Similarly, if the peripheral region 82 is of less interest, the thirdand fourth emission points 86, 88 may be low quality, such as low flux,X-ray tubes or emitters.

As with the preceding examples, because the entire field of view 72 isnot covered by a single emission point 70 and detector 22, the in-planesize of the detector 22, such as flat-panel detector 60, may be smallerthan if a single emission point 70 were employed. Similarly, half-scandata acquisition using the first and second emission points 74, 76 maybe used to acquire data for reconstructing the central region ofinterest 80, i.e., 180°+ some additional angle of rotation. However, afull rotation, i.e., 360°, of the first, second, third, and fourthemission points 74, 76, 86, 88 may be needed to acquire data forreconstructing the full field of view 72, i.e., to fully reconstruct theperipheral region 82.

While the preceding example depict configurations employing two or fouremission points 70, one of ordinary skill in the art will appreciatethat the disclosed techniques extend to other configurations in whichmore than one emission point 70 is present. Similarly, field of viewconfigurations other than those depicted are not excluded from thepresent technique and may benefit from the use of multiple emissionpoints 70, as discussed herein.

Furthermore, it may sometimes be desirable to offset the emission points70 in the z-direction. For example, as shown in FIG. 8, a z-offset maybe applied to consecutive emission points 70, resulting in a slightlytilted arc, relative to the primary axes of the CT scanner 100, ofemission points 70. This may be particularly useful for helicalcone-beam acquisitions, because the resulting dataset may be reorderedto emulate an acquisition obtained with a single emission point. Toachieve such a result, the z-offsets, and therefore the pitch of theresulting arc, will depend on the helical pitch employed during imageacquisition. The z-offsets may be adjusted to accommodate a desiredhelical pitch.

In addition, for cone-beam and volumetric CT geometries, it may bedesirable to include additional emission points 70 along thelongitudinal axis. In particular, the use of multiple emission points 70along the longitudinal axis may allow the axial extent of the detector22 to be reduced instead of or in addition to the reduction of thein-plane extent of the detector discussed above. For example, referringto FIG. 9, three emission points 70 deployed along the longitudinal axisof a CT scanner 100 are depicted. The emission points 70 may be firedalternatingly, such as sequentially, so that only one emission point 70is active at a time. A detector 22, such as flat-panel detector 60, witha reduced axial extent may be employed in conjunction with the multiplelongitudinal emission points in a manner analogous to that discussed inthe preceding examples. As in the preceding examples, implementations ofthe present technique longitudinally allow for the use of smaller coneangles and therefore smaller detectors 22 longitudinally.

For example, referring to FIG. 10, three sets of duplicate emissionpoints 94, 96, 98 are depicted along the longitudinal axis of a CTscanner 100. In the depicted example, each set of duplicate emissionpoints 94, 96, 98 share coordinates within the y-plane, but differ intheir position on the z-axis, i.e., longitudinally.

As described in the preceding in-plane offset and longitudinal offsetexamples, the techniques disclosed herein may provide a variety ofbenefits. For example, the reduced in-plane and/or longitudinal extentof the detector 22 may allow smaller, less expensive detectors, such asflat-panel detectors 60, to be employed (FIGS. 2-7 and 9). In general,it is easier and less expensive to manufacture a smaller detector,particularly a flat-panel detector.

In addition, the present techniques may provide greater spatialresolution, particularly away from the isocenter. In particular, asingle emission point may be associated with a large fan angle and acorrespondingly large detector. The focal spot associated with theemission point looks bigger at the edge of the detector due to anincrease in the so-called apparent focal-spot size. The increasedapparent focal-spot size may lead to inferior spatial resolution at theedges of the detector compared to the center of the detector. Thereduced fan angles and smaller in-plane extent of detectors 22 used inconjunction with the present technique (FIGS. 2-7 and 9) may allowspatial resolution to be improved away from the isocenter, i.e., overthe rest of the field of view, due to the smaller apparent focal size ofthe emission points 70.

Furthermore, the use of multiple emission points 70 (FIGS. 2-7) mayallow for dynamic flux control during an image acquisition. For example,the multiple emission points 70 may be differentially activated based onview angle to maintain uniformity of the signal at the detector 22 and,thereby, improve efficiency and limit the dynamic range at the detector,or in order to optimize the dose or image quality. In particular, inmedical imaging contexts, the patient 18 (FIG. 1) typically iselliptical in cross-section, resulting varying path lengths through thepatient 18, i.e., the path length an X-ray traverses through the patient18 varies depending on the view angle position relative to the patient18. Conventional CT techniques may employ a bowtie filter, adapted tothe general cross-section of the body region being imaged, to compensatefor these varying path lengths.

The present techniques, however, allow for the real time flux modulationbased on the anatomy of the patient 18, i.e., a virtual dynamic bowtie.In particular, at view angles corresponding to a short path lengththrough the patient 18, such as through the chest and back, an emissionpoint 70 may be activated to emit X-rays having lower flux. Conversely,at view angles corresponding to a long path length, such as fromshoulder to shoulder, an emission point 70 may be activated to emitX-rays having higher flux. Similarly, for intermediate path lengths, theflux of the emitted X-rays may be suitably adjusted. Furthermore, theflux associated with a view angle position may be dynamically adjustedas a patient is linearly displaced through the CT scanner. In thismanner, the effects of a bowtie filter may be replicated while allowingdynamic adjustment to maintain uniformity of signal at the detector 22.

The present techniques may also allow for the use of various detectortechnologies, such as energy discrimination detectors, so that CTtechniques such as energy discrimination CT may be performed. Because ofthe smaller detector extent in the in-plane and/or longitudinaldirections, such exotic technologies may more affordably be implemented.Similarly, such detectors may also be more easily manufactured toaccommodate the reduced detector dimensions associated with the presenttechniques. In addition, the smaller fan angles and cone anglesassociated with the present technique reduce scatter in the X-rayintensity measurements and may allow the anti-scatter grid to be omittedfrom the detector, thereby increasing detector efficiency.

While the invention may be susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and have been described in detail herein.However, it should be understood that the invention is not intended tobe limited to the particular forms disclosed. For example, thoughimaging in a medical context has been discussed, the present techniquesmay also be applied in other imaging contexts, such as the screening ofbaggage, packages, and passengers. Rather, the invention is to cover allmodifications, equivalents, and alternatives falling within the spiritand scope of the invention as defined by the following appended claims.

1. A CT system comprising: two or more X-ray sources radially offsetfrom one another about an imaging volume and configured to rotate aboutthe imaging volume, wherein each X-ray source comprises a separate anddiscrete emitter of electrons, and when activated, each X-ray sourceilluminates substantially different radial portions of the imagingvolume; a controller configured to control operation of the two or moreX-ray sources; and a detector configured to rotate about the imagingvolume and to generate signals in response to emitted X-rays.
 2. The CTsystem of claim 1, wherein each of the discrete emitters is one of athermionic emitter, a field emitter, or a dispenser cathode.
 3. The CTsystem of claim 1, wherein the controller is configured to controloperation of the two or more X-ray sources such that the two or moreX-ray sources differentially emit X-rays.
 4. The CT system of claim 1,wherein the detector is a flat panel detector.
 5. A CT systemcomprising: two or more radially offset emission points configured torotate about an imaging volume, wherein each emission point, whenactive, emits X-rays in fan beams or cone beams that are substantiallynon-overlapping where they enter a field of view of the imaging volume;a controller configured to control operation of the two or more emissionpoints; and a detector configured to rotate about the imaging volume andto generate signals in response to the X-rays.
 6. The CT system of claim5, wherein the X-rays emitted by at least one offset emission point passthrough the isocenter of the field of view and the X-ray emitted by atleast one different offset emission point do not pass through theisocenter.
 7. The CT system of claim 5, wherein two or more offsetemission points comprise two offset emission points, three offsetemission points, or four offset emission points.
 8. The CT system ofclaim 5, wherein the controller is configured to differentially operatethe two or more offset emission points.
 9. The CT system of claim 5,wherein the detector is a flat panel detector.
 10. A CT systemcomprising: two or more emission points configured to rotate about afield of view and, when rotated, to each emit X-rays that pass throughdifferent radial regions of the field of view, wherein each of the twoor more emission points comprises a separate and discrete emitter ofelectrons; a controller configured to differentially operate the two ormore emission points; and a detector configured to generate signals inresponse to the X-rays.
 11. The CT system of claim 10, wherein thecontroller differentially operates the two or more emission points byoperating at least one emission point at a different electric current ormA than another.
 12. The CT system of claim 10, wherein the controllerdifferentially operates the two or more emission points by activating atleast one emission point for a greater duration than another.
 13. The CTsystem of claim 10, wherein the controller differentially operates thetwo or more emission points by operating at least one emission pointmore frequently than another.
 14. The CT system of claim 10, wherein thetwo or more emission points are associated with one or more X-ray tubes,one or more solid-state X-ray sources, or one or more thermionic X-raysources.
 15. A method comprising: rotating two or more radially offsetemission points about an imaging volume; differentially emitting X-raysfrom the two or more emission points as the emission points rotate, suchthat a patient's dose is minimized; and generating signals in responseto the incidence of the X-rays on a detector.
 16. The method of claim15, wherein differentially emitting X-ray comprises operating at leastone emission point at a different flux than another.
 17. The method ofclaim 15, wherein differentially emitting X-ray comprises activating atleast one emission point for a greater duration than another.
 18. Themethod of claim 15, wherein differentially emitting X-ray comprisesoperating at least one emission point more frequently than another.